opencv imread parameters

    默认值为1：即
    

    always convert image to the 3 channel BGR color image
    

    enum ImreadModes {
    

    IMREAD_UNCHANGED            = -1, //!< If set, return the loaded image as is (with alpha channel, otherwise it gets cropped).//原始图像有alpha通道的话，就保存该通道数据，否则，别的flags会直接将alpha通道丢弃。

    
    IMREAD_GRAYSCALE            = 0,  //!< If set, always convert image to the single channel grayscale image.
    
    IMREAD_COLOR                = 1,  //!< If set, always convert image to the 3 channel BGR color image.
    
    IMREAD_ANYDEPTH             = 2,  //!< If se